DC offset is a common problem in direct-conversion receivers. DC offset in the received signal may be caused by several factors, such as self mixing at the receiver's RF mixers, calibration residue at the receiver's analog components and carrier leakage from the transmitter. This DC offset is typically less of a problem in OFDM systems because the DC tone is not used for signal transmission. However, there may still be a need to reduce or eliminate DC offset in an OFDM system in order to improve system performance. For example, reducing or cancelling DC offset may reduce the headroom requirement for each module in the analog and data path, prevent clipping during FFT processing due to DC accumulation, and reduce interference leakage from DC tone to other adjacent tones.
Traditionally, DC offset in a received communication signal is calibrated by applying a high pass filter after analog-to-digital conversion. With this method, the performance of DC calibration is dependent on the high-pass corner frequency of the filter. A high corner frequency will add more distortion to the signal, but will require less time to remove the DC offset. A lower corner frequency will result in less distortion, but will take longer to remove the DC offset. Consequently, the receiver either suffers from signal distortion by using a fast high pass filter or suffers from strong DC interference leakage during the first few symbols of a frame by using a slower high pass filter. This tradeoff often proves to be extremely challenging for system designers.